1. Field of the Invention
This invention relates to a wavelength detection device for a line-narrowed laser apparatus and to a line-narrowed laser apparatus. More specifically, the present invention relates to a wavelength detection device that is ideal for detecting wavelengths in line-narrowed laser light spectrums. The present invention also relates to an ultra line-narrowed fluorine laser apparatus that narrows the line of the laser light of a fluorine laser and provides it as an exposure light source for an exposure apparatus.
2. Description of the Related Art
In cases where laser light is used as the light source in a stepper (reduction projection exposure device), it is necessary to narrow the line of the laser light spectrum by a line narrowing element such as an etalon or grating.
It is also necessary that the center wavelength in the spectrum of this line-narrowed oscillation line be stabilized and controlled with high precision so that there is no divergence during exposure.
In FIG. 27 is diagrammed a common laser wavelength stabilizing control device.
The line narrowing and wavelength selecting are performed by driving an etalon 3 that is a line narrowing element by a wavelength controller 11 through a driver 10 (regulating the installation angle of the etalon 3), and driving a fully reflective mirror 8 by the wavelength controller 11 through a driver 9a (regulating the installation angle of the fully reflective mirror 8).
The wavelength is controlled so that the center wavelength of the narrowed oscillation line L0 does not fluctuate during the exposure.
That is, during the exposure, the absolute wavelength of the line-narrowed oscillation line L0 is detected by detecting the relative wavelength of the line-narrowed oscillation line L0 relative to a constant reference beam Lx.
In other words, the laser beam output from a reference light source 32 is input as the reference light Lx to a spectroscope 12. The narrowed oscillation line L0 for which it is desired to detect the wavelength is simultaneously input, as the light to be detected L0, via beam splitters 13 and 14, to the same spectroscope 12. In the spectroscope 12, the reference light Lx and the light to be detected L0 are subjected to spectral diffraction, and an image of the diffracted light is formed on a line sensor 20. The detection position on the line sensor 20 corresponds to the detected wavelength.
Then, using a dispersion value, from the difference in the positions detected on the line sensor 20, the relative wavelength of the light to be detected L0 relative to the reference light Lx is found, whereupon, based on that found relative wavelength and the known wavelength of the reference light Lx, the absolute wavelength of the light to be detected L0 is calculated.
These calculation results are next fed back to the wavelength controller 11, and, thereby, the etalon 3 is driven by the driver 9e. 
The center wavelength of the narrowed oscillation line. L0 that is made to oscillate between the fully reflecting mirror 8 and the output mirror 4 through the laser chamber 1 and etalon 3 is then fixed as the targeted wavelength.
In this manner, stabilizing control is effected with high precision so that the center wavelength in the narrowed oscillation line L0 does not diverge during exposure.
With the conventional laser wavelength stabilizing control device, however, a problem is incurred in that the structure becomes complex due to the necessity of the reference light source 32 for outputting the reference light Lx, as described above. When the wavelength of the narrowed oscillation line L0 is detected with high precision, furthermore, a problem is incurred in that the light intensity of the laser beam output by the lamp used for the reference light source is low.
Thereupon, in Japanese Patent Application Laid-Open No. 5-95154, as published, for example, an invention is described wherewith, when the narrowed oscillation line L0 is a molecule fluorine F2 laser beam, an atom fluorine laser beam is used having a wavelength in the visible region.
Based on the invention described in this publication, it is possible not to provide a reference light source in the wavelength stabilizing control device.
With the invention described in the publication noted above, the wavelength of the fluorine atom laser oscillation line used as the reference light Lx is in the visible light region. That is, the wavelength of an atom fluorine laser beam is in a region that is removed from the vacuum ultraviolet region that contains the wavelength of a molecule fluorine laser.
For this reason, when the narrowed oscillation line L0 is a molecule fluorine laser beam, the precision wherewith the wavelength of the narrowed oscillation line L0 is detected will decline when detected on the basis of the wavelength of the molecule fluorine laser beam.
In other words, with the invention described in the publication noted above, a problem is incurred in that it is very difficult to effect stabilizing control with high precision on the center wavelength of the spectrum of the narrowed oscillation line L0.
With the invention described in the publication noted above, moreover, a dielectric multilayer film mirror is employed for causing fluorine atom laser light and molecule fluorine laser light to oscillate simultaneously, providing a resonator for causing the fluorine atom laser light to oscillate inside the resonator for causing the molecule fluorine laser light to oscillate, for example.
With such a mirror, the number of layers becomes large, and a film material must be used which exhibits high absorbency for light having a wavelength of 157 nm, wherefore problems are incurred in that the molecule fluorine laser light oscillation efficiency becomes poor, and the output of the narrowed oscillation line L0 from the molecule fluorine laser light declines.
A first object of the present invention, which was devised with the situation described in the foregoing in view, is to improve the precision wherewith the wavelength of a narrowed oscillation line is detected, without using a reference light source, and without causing a decline in the narrowed oscillation line output.
Now, in terms of the performance demanded in an exposure tool used in lithography, there are many different factors, such as resolution, alignment precision, processing power, and equipment reliability. Among these factors, the resolution R that directly impacts pattern fineness is expressed by the formula R=kxc2x7xcex NA (where k is a constant, xcex is the exposure light wavelength, and NA is the numerical aperture of the projection lens). Accordingly, the shorter the exposure light wavelength xcex the better in the interest of obtaining good resolution.
Thereupon, in a conventional exposure tool, a mercury lamp i line (wavelength=365 nm) or a krypton-fluoride (KrF) excimer laser having a wavelength of 248 nm is used as the exposure tool light source. These are called an i-line exposure tool and KrF exposure tool, respectively. For the projection optical system employed in such an i-line exposure tool or KrF exposure tool, a reduction projection lens unit wherein a larger number of quartz glass lenses are assembled together is widely used.
As a next-generation exposure tool for performing ultra-fine processing, moreover, use is beginning to be made of exposure tools which employ an argon-fluoride (ArF) excimer laser having a wavelength of 193 nm for the exposure light source. These are called ArF exposure tools. In the ArF exposure tool, an ArF excimer laser is used which has its line-narrowed down to a wavelength width of approximately 0.6 pm, and an achromatic lens made of two types of material is used in the reduction projection optical system.
For the next generation of lithographic exposure tools for the ArF exposure tools described above, furthermore, research is being done on fluorine exposure tools wherein a fluorine laser having a wavelength of 157 nm is used for the light source.
In this fluorine laser, there are two oscillation lines (also called oscillation lines) having different wavelengths and light intensities. The two wavelengths are xcex1=157.6299 nm and xcex2=157.5233 nm, respectively, with the wavelength width of each oscillation line the to be on the order of 1 to 2 pm.
In order to use this fluorine laser as exposure light, it is generally believed to be advantageous to select only one line having greater intensity (xcex1=157.6299 nm) (hereinafter called single-line implementation). For this single-line implementation, conventionally, one or two prisms are used.
Furthermore, double-line fluorine laser implementation is described, for example, in xe2x80x9cCAN. J. PHYS. VOLUME. 63, 1985, pp 217-218.xe2x80x9d
Also, the results of experimentation in single-line fluorine laser implementation are reported, for example, in SPIE, 24th International Symposium on Microlithography, February 1999.xe2x80x9d
In the conventional fluorine exposure tools noted in the foregoing, however, it becomes very difficult to employ the refractive type reduction projection optical systems based solely on lenses that had been commonly used in exposure tools theretofore (that is, until the development of the ArF exposure tools). It is to be necessary to use instead a reflective-refractive type (also called a catadioptric type) that is effective against chromatic aberration.
The reason therefor is that, at a wavelength of 157 nm, the transmittance in quartz glass becomes extremely low, so that only a very limited number of materials such as calcium fluoride can be used.
For that reason, when a reduction projection lens is configured using a monochromatic lens consisting only of calcium fluoride, the level of line narrowing is inadequate even when the fluorine laser is implemented in single line.
Therefore, in reality, it is said that it is necessary to further narrow the band for that single line to a tenth or so of the wavelength width (to approximately 0.2 pm).
Furthermore, in a scheme wherein the single line of a fluorine laser is used as it is, the line spectrum is something that is absolutely established optically, wherefore, while there is no need to stabilize the wavelength, when the single line is subjected to line narrowing down to a wavelength width of approximately 0.2 pm or so, it is necessary to effect stabilization so that the line-narrowed wavelength does not flutter within the spectrum of the single line having a wavelength width of 1 to 2 pm.
Conventionally, however, it is very difficult to use other light sources or absorption lines that have a wavelength that is stabilized in the vicinity of the 157 nm wavelength, wherefore it has been very difficult to stabilize the wavelength of line-narrowed laser light.
A second object of the present invention is to further narrow the single line having a wavelength width of 1 to 2 pm while also stabilizing the wavelength of that line-narrowed laser light in a simple manner without using a reference light source.
Thereupon, a first invention, for attaining the first object noted earlier, is a wavelength detection device for detecting, on the basis of a wavelength of a reference light, a wavelength of a narrowed oscillation line output from a line-narrowed laser apparatus in which are deployed a laser medium and a line narrowing element, wherein:
of non-line-narrowed spontaneous emission beams emitted from the laser medium, a spontaneous emission beam whose wavelength approximates the narrowed oscillation line and whose light intensity is a certain level or higher, is used as the reference light.
A second invention, for attaining the first object noted earlier, is a wavelength detection device for detecting, on the basis of a wavelength of a reference light, a wavelength of a narrowed oscillation line output from a line-narrowed oscillating molecule fluorine laser apparatus in which are deployed a line narrowing element and a laser chamber for emitting molecule fluorine emission beams, wherein:
the wavelength detection device comprises emission beam detection means for detecting non-line-narrowed molecule fluorine emission beams emitted from the laser chamber; and
one, or, alternatively, two of the molecule fluorine emission beams detected by the emission beam detection means are used as the reference light.
The first invention and the second invention cited above are described in correspondence with FIG. 1, FIG. 2, and FIG. 8.
That is, based on the first invention and the second invention, prior to the line narrowing of the molecule fluorine emission beams L1 and L2, for example, the molecule fluorine emission beams L1 and L2 are detected, and one or, alternatively, two of those detected molecule fluorine emission beams L1 and L2 are used as the reference light L.
In the-first invention and the second invention, as noted above, provision is made so, that emission beams L that have not been subjected to line narrowing are detected, and, of those detected emission, beams L, the emission beam L the wavelength whereof approximates the narrowed oscillation line L0 and the light intensity whereof is at or higher than a certain level is used as the reference light. Therefore the precision wherewith the wavelength of the narrowed oscillation line is detected can be improved without reducing the output of the line-narrowed oscillating laser.
A third invention, for attaining the first object noted earlier, is a wavelength detection device for detecting, on the basis of a wavelength of a reference light, a wavelength of a narrowed oscillation line output from a line-narrowed oscillating molecule fluorine laser apparatus in which are deployed a line narrowing element and a laser chamber for emitting molecule fluorine emission beams, wherein:
the wavelength detection device comprises:
time setting means for setting a certain time period; and
light interruption means for interrupting non-line-narrowed molecule fluorine emission beams emitted from the laser chamber for the certain time period set by the time setting means; and wherein:
one or, alternatively, two of the molecule fluorine emission beams interrupted by the light interruption are used as the reference right.
The third invention cited above is described in correspondence with FIG. 2, FIG. 10, and FIG. 12.
That is, based on the third invention, a certain time period is set, the molecule fluorine emission beams L1 and L2 are interrupted for the certain time period that is set before they are input to a line narrowing element 3, and one or, alternatively, two of the molecule fluorine emission beams L1 and L2 at the time of being interrupted are used as the reference light L.
Thus, in this third invention, provision is made so that a certain time period is set, the molecule fluorine emission beams L1 and L2 are interrupted for the certain time period that is set before they are input to a line narrowing element 3, and one or, alternatively, two of the molecule fluorine emission beams L1 and L2 at the-time of being interrupted are used as the reference light L. Therefore the precision wherewith the wavelength of the narrowed oscillation line is detected can be improved without using a reference light source and without lowering the output of the narrowed emission beams.
A fourth invention, for attaining the first object noted earlier, is a wavelength detection device for detecting, on the basis of a wavelength of a reference light, a wavelength of a narrowed oscillation line output from a line-narrowed oscillating molecule fluorine laser apparatus in which are deployed a line narrowing element and a laser chamber for emitting molecule fluorine emission beams, wherein:
the wavelength detection device comprises:
time setting means for setting a certain time period; and
installation angle changing means for changing, during the certain time set by the time setting means only, an installation angle of the line narrowing element, from an installation angle at which spectrum of output beam of the molecule fluorine laser is line-narrowed to an installation angle at which the molecule fluorine laser oscillation line is not line-narrowed; and wherein:
one or, alternatively, two of the molecule fluorine laser emission beams output from the laser chamber are used as the reference light when the installation angle of the line narrowing element has been changed by the installation angle changing means to the installation angle whereat the molecule fluorine laser emission beam is not line-narrowed.
The fourth invention cited above is described in correspondence with FIG. 2, FIG. 14, and FIG. 16.
That is, based on this fourth invention, a certain time period is set, and the installation angle of the line narrowing element 3 is changed, for the set certain time, from an installation angle whereat the molecule fluorine laser emission beams L1 and L2 are line-narrowed to an installation angle whereat the molecule fluorine laser emission beams L1 and L2 are not line-narrowed, and, at that time, one or, alternatively, two of the molecule fluorine laser emission beams L1 and L2 output from the laser chamber 1 are used as the reference light L.
Thus, with this fourth invention, provision is made so that a certain time interval is set, and one or, alternatively, two of the molecule fluorine laser emission beams L1 and L2 output from the laser chamber 1 are used as the reference light L when the installation angle of the line narrowing element 3 is changed, during the certain set time period, from an installation angle whereat the molecule fluorine laser emission beams L1 and L2 are line-narrowed to an installation angle whereat the molecule fluorine laser emission beams L1 and L2 are not line-narrowed. Therefore the precision wherewith the wavelength of the narrowed oscillation line is detected can be improved without using a reference light source and without lowering the output of the narrowed emission beams.
A fifth invention, for attaining the second object noted earlier, is an ultra line-narrowed fluorine laser apparatus for line narrowing laser light which is laser-oscillated, comprising a laser chamber for laser-oscillating a fluorine laser; and a first resonator for causing the laser light oscillated by the laser chamber to resonate; wherein:
the ultra line-narrowed fluorine laser apparatus further comprises:
a line narrowing element for line narrowing and outputting one of two oscillation lines having different wavelengths and light intensities in the laser-oscillated laser light;
a second resonator for causing the one oscillation line to oscillate without being line-narrowed;
detection means for detecting a difference between a center wavelength in spectrum of laser light output from the line narrowing element and a center wavelength in spectrum of laser light oscillated from the second resonator; and
control means for controlling the line narrowing element so that the difference in the center wavelengths in spectrums of the two laser lights detected by the detection means falls within an allowable range.
A sixth invention is the fifth invention wherein: the detection means comprises a spectroscope for receiving laser light output from the line narrowing element and laser light oscillated by the second resonator, and measuring spectrums of those two laser lights; and
the spectroscope detects a wavelength of spectrum of laser light output from the line narrowing element, also detects a wavelength of spectrum of laser light oscillated from the second resonator, and detects the difference between the center wavelengths of these two spectrums.
And a seventh invention is the sixth invention wherein the spectroscope has a scanning Fabry-Perot etalon.
The fifth to seventh inventions are next described with reference to FIG. 18 and FIGS. 19(a) and 19(b).
As diagrammed in FIG. 18, in an ultra line-narrowed fluorine laser apparatus 600, a first resonator is configured with a fully reflecting mirror 8 and an output mirror 4, while a second resonator is configured with a fully reflecting mirror 32 and the front surface P of a prism 33a. 
The first resonator is a stabilized type, and within this stabilized type of first resonator is deployed a laser chamber 1.
In the first resonator, an etalon 56 is deployed between the fully reflecting mirror 8 and the laser chamber 1, and oscillation lines are further line-narrowed (that is, ultra line-narrowed) by this etalon 56.
In the etalon 56, centered on a strong line having a wavelength xcex1=1.57.6299 nm, the maximum transmittance wavelengths thereof are matched, wherefore, when laser oscillation is induced, a laser beam L10 that is ultra line-narrowed to a wavelength width of approximately 0.2 pm at the wavelength xcex1=157.6299 is obtained from the output mirror 4.
Meanwhile, the second resonator is a stabilized type, wherein, because no line narrowing element is contained in this second resonator, the two fluorine laser lines are oscillated as is.
The Laser beam L12 containing the two lines output from this second resonator, while advancing through the two prisms 33a and 33b, will have two slight angular differences arise in the direction of advance of the two lines, respectively, due to wavelength dispersion.
Here, in the laser light containing the two lines reflected to the mirror 35, a laser beam L13 having one line (line having wavelength xcex1=157.6299 nm) is set so that it passes through a pinhole 34b. 
The spectroscope 37 is configured by a scanning Fabry-Perot etalon (not shown) and a piezo element (not shown) that changes the gap interval in that etalon. The spectroscope 37 inputs both the ultra line-narrowed laser beam L11 having the wavelength xcex1 of 157.6299 nm and the laser beam L13 that is not ultra line-narrowed having the wavelength xcex1 of 157.6299 nm, detects the spectrums of those laser beams, respectively (cf. FIGS. 19(a) and 19(b)), and sends the detection results to a control unit 40.
The control unit 40, based on the detection results from the spectroscope 37, controls the turning of a turning stage 41 via a signal line 39b, in order to stabilize the center wavelength in the spectrum of the ultra line-narrowed laser beam L11, so as to cause the center wavelength in the wide spectrum of the laser beam L13 that is not ultra line-narrowed (i.e. the absolute wavelength thereof) to coincide with the center wavelength in the spectrum of the ultra line-narrowed laser beam L11.
As described in the foregoing, based on the fifth and sixth inventions, control is effected so that the difference between the center wavelength in the spectrum of the laser beam that is not ultra line-narrowed and the center wavelength in the spectrum of the ultra line-narrowed laser beam falls within an allowable range that is set beforehand (that is, so that the center wavelengths of the two spectrums coincide, for example). Therefore, the center wavelength in the spectrum of the ultra line-narrowed laser beam can be definitely stabilized without using a reference light source.
Based on the seventh invention, furthermore, a spectroscope provided in the detection means for detecting the spectrum of laser light for wavelength stabilization is made so that a scanning Fabry-Perot etalon is used. Compared to a spectroscope wherein a diffraction grating is used, therefore, the spectroscope can be made more compact, and a laser apparatus into which this spectroscope is incorporated can also be made more compact.